While Labor Day is the unofficial end of summer , it 's also the unofficial start to the campaign season .

That means politicking will be on the rise , especially as control of the Senate is at stake as well as control of 36 state houses . So , if you turn on your TV , expect to see more -- and nastier -- political advertisements . In fact , Elizabeth Wilner , senior vice president of Kantar Ad Intelligence , says as much as $ 3.4 billion is going to be spent on advertising this midterm season .

The race with the most at stake is the one for U.S. Senate in Kentucky . This is not just one of the only Senate races Republicans are at risk of losing , but also the race where the top Senate Republican is at risk of losing his job .

The Republican leader of the Senate , Mitch McConnell , is in a fight for his political life against Alison Lundergan Grimes , a relatively inexperienced Democratic politician who was just 8 years old when McConnell started his first term in the Senate in 1985 .

McConnell , a shrewd politician , prolific fundraiser and expert campaigner , has had numerous missteps , making this race even more interesting . His troubles include a flubbed campaign ad , a campaign manager who was a little too honest , the recent resignation of that manager and a caught-on-tape moment .

Grimes , meanwhile , has also stumbled when talking about foreign policy , and questions have arisen about a possible sweetheart deal involving her campaign bus .

This is one of the most interesting and critical races in the country .

How Mitch McConnell crushed the tea party

Here are four other races that are worth watching :

Wisconsin governor : The Wisconsin governor 's race has many national repercussions , as Republican Gov. Scott Walker is locked in a tight re-election battle against former Trek bicycle executive Mary Burke .

Economic policy is a central component of this campaign .

Walker has gained prominence in conservative politics for governing as a fiscal conservative and making deep cuts to spending by cutting public union workers ' pensions . He also drastically limited workers ' bargaining rights . Meanwhile , Democrats , backed by labor unions , are again fighting to defeat Walker -- they forced a recall two years ago that Walker won -- to move forward on more Democratic economic policies , including lifting the minimum wage .

Walker , who is also being investigated for alleged illegal campaign coordination with outside groups , is considered a potential 2016 Republican presidential candidate , but if he loses his gubernatorial race , his path to the presidency will be very , very narrow .

Democrats and Republicans understand the stakes , and President Barack Obama traveled to Wisconsin on Labor Day to speak at a union event in a trip packed with political symbolism .

Walker , Burke tied up in new poll

U.S. Senate , Louisiana : Democratic Sen. Mary Landrieu always has tough races , and her fourth bid for the Senate seat is no exception . Republican Rep. Bill Cassidy is the person who is giving Landrieu another difficult run .

Cassidy is tying Landrieu to Obama in this conservative-leaning state while painting her as a corrupt Washington insider . Landrieu , meanwhile , is attempting to paint her Senate tenure as a picture of independence .

Most interestingly , a political roller coaster is possible . If Landrieu or Cassidy does n't receive more than 50 % of the vote on Election Day in November , a winner wo n't be named until that state 's December 6 election . There 's a chance the Louisiana race , and the balance of the Senate , might be dragged out until December .

Sen. Mary Landrieu to reimburse Senate for charter flight

National Democrats go after Cassidy on Medicare

U.S. Senate , Iowa : When popular Sen. Tom Harkin decided to retire , Democrats had a small panic attack as this solidly Democratic seat was now in play . But when Rep. Bruce Braley jumped in , Democrats ' confidence was restored .

But then that confidence has been shaken as Braley has run a gaffe-prone campaign that involves digs at farmers and meandering chickens . His missteps -- combined with the surprising strength of Republican candidate Joni Ernst , who has run a great campaign that began with a breakout performance in the crowded Republican primary -- make this a possible and unexpected pickup for Republicans .

Climate group attacks Ernst on tax pledge , not climate

Florida 's 2nd Congressional District : There is little to no chance that Republicans will lose control of the House of Representatives , and some race analyzers say the GOP will even pick up seats . But this race could be a bright spot for Democrats .

Even in what is expected to be a difficult year for Democrats , Democratic candidate Gwen Graham could pull out a victory in this Republican-leaning district of Tallahassee and the central part of the Florida Panhandle .

Graham has some advantages . She has no problem getting money -- raising more than Southerland -- and she has a Florida-famous last name . Her father is longtime Sen. Bob Graham , giving her access to his connections and deep knowledge of running successful campaigns .

Key races in 2014

Complete midterm coverage

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Labor Day is the unofficial end of summer and the unofficial start to campaign season

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As much as $ 3.4 billion could be spent on advertising for this midterm election

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Here are five must-follow races for these midterms